The towed suction basket described in German Patent Publication DE 14 84 812 C3 has several water pressure nozzles disposed at a close distance from each other above a waterbed for directing water under pressure onto the solid material thereon, and a rear suction gap formed by the rear edge of a backwardly extending hood, which is freely pivotal around a horizontal axis transverse to the tow direction of a towed suction basket. The width of the suction gap is determined by sliding blocks on the rear edge of the hood. Because of the above construction, the towed suction basket can rest in a sealing manner on the waterbed on its front edge while a pressure jet of water is directed down to a desired depth into the solid material on the waterbed by means of the row of the pressure nozzles. As a result of the above, the solid material on the waterbed is loosened, dislodged, and dispersed in the area of the suction gap.
The above-described towed suction basket and of the method associated with its use has the disadvantage of creating an unfavorable ratio of the amount of suctioned solid material to the conveyed amount of water, which ratio is approximately 1:3. A considerable amount of water pressure at a pressure of approximately 6 bar must be maintained in order to loosen satisfactory amounts of the solid material on the waterbed.
German Patent Publication DE 24 48 308 C2 discloses the use of a suction pump located in the suction line, as well as a pressure pump located in the pressure water line to the towed suction basket, for suctioning the interior of the hopper, so that the water suctioned with the solid material can be returned to the vicinity of an outlet for the suction line to serve as feed medium for the solids to be suctioned. The aspiration of the overflow water is performed via a suction basket, whose height in the hopper or the box frame is set to correspond to the desired filling level at that location.
Prior art constructions concerning the loading and unloading of the hopper in a hopper suction dredger lead to other disadvantages, as will be described below.
First, during loading of the hopper, it is necessary to ensure that water is removed to the greatest extent possible from the suctioned solids-water suspension, a large portion of which consists of water, in order to allow for optimum use of the loading capacity of the hopper. For this purpose, the mixture conveyed by the dredger pumps is introduced at one end of the hopper via U-pipes or open channels, after which the mixture flows through the length of the hopper. During this process, the dredged solid material can settle and the feed water can be diverted into the ocean via height-adjustable overflow dams to the extent that it has not been suctioned off, as described above. The above notwithstanding, any solid material with finer grains will still remain in the feed water flowing or being pumped out, creating so-called overflow losses, which are essentially a function of the flow-through speed in the hopper.
To minimize overflow losses, it has already been proposed to introduce the solids-water suspension into the hopper through a box frame disposed in the center of the dredging vessel or dredger, with overflow mechanisms located at both longitudinal ends of the hopper. As a result of the above construction, the mean flow-through speed through the hopper is theoretically cut by half, so that the deposition conditions for the solids carried along are improved and the loading time can be correspondingly shortened. Overflow losses can be reduced by approximately one third according to the above construction.
To empty the hopper of solids filled therein, it is necessary to first render the solids flowable once more. The above is accomplished by supplying water to the surface of the solids, or by the introduction of water via nozzles in the bottom of the hopper, or by like methods. To accomplish the above, German Patent Publication DE 24 57 020 C3 proposes moving the flowable solids-water suspension off the hopper via a height-adjustable overflow device disposed in a separate chamber, or to let the solids-water mixture flow out of the bottom of the hopper, which is inclined in the longitudinal direction of the vessel, and which is provided, on it lowest point, with a closeable bottom opening. As a controllable flow-off dam, the overflow device can consist of a plurality of flaps, or annular dams, or dam rings disposed on top of one another.